1. Field of the Invention
The present invention relates to a lithographic projection apparatus.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a pattering device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a pattering device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
The manufacture of ICs and other devices with such apparatus generally involves the replication of extremely fine sub-micron patterns, with an exceptionally high degree of positional accuracy. For this reason, it is essential to properly isolate various critical parts of the apparatus (such as the substrate table and mask table, for example) from spurious motion, vibration, mechanical shocks, etc. In general, this is achieved using such measures as carefully designed metrology frames, air-mounts, motional counterweights and dampers, which serve to isolate the apparatus"" critical parts from most unwanted mechanical influences. However, such measures are not completely effective in eliminating a number of unwanted influences, such as, for example:
1. vibrations in the substrate table due to leveling actions during exposure;
2. vibrations caused by motion of reticle masking blades;
3. resonance effects caused by the presence of air showers;
4. vibrations in the substrate table caused by motion of the mask table, and vice versa, and;
5. influence of air shower flow on the substrate table.
Although these effects are relatively small, they become increasingly important as the need to produce ever-higher device resolutions increases, and they now form a substantial barrier to the viable realization of large-area ICs having critical dimensions of the order of 0.15 xcexcm and less.
Accordingly, it has been proposed in U.S. Pat. No. 6,373,072, incorporated herein by reference, to provide a control system for the substrate and mask tables of a lithographic apparatus in which errors in the position of the substrate table are compensated for by their inclusion as a feed-forward control in the mask table control loop. Specifically, the substrate table error is lowpass filtered, the output of the filter is then added to the mask table setpoint, and also twice differentiated, multiplied by the mask table mass and the resultant force applied to the mask table. This proposal is based on the realization that the absolute positions of the mask and substrate tables are less important than their relative position and allows the correction of substrate table errors beyond the mask table bandwidth. However, this control system has performance limits, in part caused by the inevitable time delay in processing the substrate table error.
It is an aspect of the present invention to provide an improved control system for the support structure for the patterning device, e.g. a mask, and substrate tables of a lithographic projection apparatus.
This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to provide a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table to hold a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate; a first driving device constructed and arranged to move the support structure in a given reference direction substantially parallel to the plane of the patterning device; a second driving device constructed and arranged to move the substrate table parallel to the reference direction so as to be synchronous with the motion of the support structure; a first measuring device constructed and arranged to determine the momentary position of the patterning device with respect to a fixed reference point; a second measuring device and constructed and arranged to determine the momentary position of the substrate table with respect to a fixed reference point; a comparing device constructed and arranged to compare the measured momentary position of the substrate table with a desired momentary position of the substrate table to generate a position error signal in accordance with a difference between the two positions; and a predictor constructed and arranged to generate a future position error signal based on one or more previous position error signals and pass that future position error signal to a correction device constructed and arranged to adjust the momentary position of the patterning device so as to compensate for such difference.
With the present invention, the drawbacks of the prior art are avoided and a improvement of 30 to 50% in the high-frequency synchronization error between the substrate and mask tables can be achieved. Preferably, the prediction is a sine-based prediction which can readily be adapted to predict the second derivative of the position error signal thereby directly providing the basis for an acceleration error signal. Such a sine-based prediction can advantageously be implemented using one or more finite impulse response (FIR) filters.
According to a further aspect of the invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system using a patterning device to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, wherein the projecting moving a support structure supporting the patterning device in a given reference direction substantially parallel to the plane of the patterning device; moving the substrate table parallel to the reference direction so as to be synchronized with the motion of the support structure; determining the momentary position of the mask table with respect to a fixed reference point; determining the momentary position of the substrate table with respect to a fixed reference point; comparing the measured momentary position of the substrate table with a desired momentary position of the substrate table to generate a position error signal in accordance with a difference between the two positions; predicting a future position error based on one or more previous position error signals; and adjusting the momentary position of the mask table so as to compensate for such difference.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.